The present invention relates to a vacuum brake force booster for a slip-controlled brake system wherein by applying a control force to a valve piston the brake valve of the vacuum brake force booster is activated without applying pedal force, thereby to modulate pressure in the brake circuits.
A vacuum brake force booster of the type to which the present invention is directed and provides an improvement is disclosed, for example, in German patent application DE OS 33 17 629. The vacuum brake force booster is part of a brake system comprising the vacuum brake force booster itself, a master brake cylinder actuated thereby and being in communication, through blockable brake conduits, with the wheel brake cylinders. In addition, it comprises a master valve to exchange the connection to the atmosphere and the vacuum connection of the vacuum brake force booster.
With the brake applied, the system works in the usual manner. Through the brake valve, air is admitted to the booster chamber of the vacuum brake force booster, thereby forcing the booster wall toward the master brake cylinder and generating a hydraulic pressure in the brake system causing the brakes to be applied.
An excessive pressure in the brake system results in wheel locking which can be detected by means of sensors. An electronic analyzer receiving the sensor signals provides a control signal to the master valve causing the vacuum connection and the connection to the atmosphere of the vacuum brake force booster to be interchanged. As a consequence, the vacuum brake force booster no longer will operate in concurrence with but rather against the pedal force, thereby reducing the pressure in the brake system. A system of the afore-described type now involves two disadvantages: firstly, provision of a complex master valve is required rendering conditional compliance with a variety of criteria, to wit, on the one hand, quick operation to insure a rapid pressure decrease within the brake system and, on the other hand, provision for large opening cross-sections is required because substantial flow volume of air will have to be accommodated within the short switching time available.
Another disadvantage resides in that the brake system cannot be readily employed for the traction slip control. A traction slip control is intended to preclude the driven wheels from racing in view of an excessive engine torque not adapted to the frictional relationship between tire and road. For this reason, irrespective of the brake actuation, a brake pressure is intended to be admitted to the brake system.
Another brake system operating on the same system as the brake system according to DE-OS 33 17 629, is known. The afore-system equally exhibits an external master valve. Moreover, the vacuum brake force booster is of a highly complex design as it comprises a total of three chambers.
DE-OS 32 40 680 discloses a brake system comprising a hydraulic brake force booster exhibiting a valve piston disposed in parallel to the booster piston to which can be applied not only the pedal force but also an electromagnetic force.
The object of the invention is to provide a vacuum brake force booster for a slip-controlled brake system, which is simple in design and eliminates the use of a master valve. Moreover, it is to enable the use of a traction slip control with a vacuum brake force booster.